I want something for my power that acts like a diode, but without the voltage drop, to protect the circuit from damage if the batteries are connected the wrong way. Is this even possible?
Electronic – Circuit Protection
protection
Related Solutions
A diode won't work anyway because the load will be connected to the battery, not the charger.
It sounds like this can happen often enough that a fuse is not a good option because it would have to be replaced too often.
This sounds like a good application for a circuit breaker. A breaker may take a few 100 ms to trip, but nothing bad should happen in that short a time. There are different types of breakers. The basic kind needs to be manually reset, but most of those have the best characteristics in terms of reaction time, on resistance, and accuracy. They usually work on the current causing a magnetic field that trips a mousetrap type of latch that opens a switch. The automatically resetting type is usually thermal with a bi-metallic element activating the switch. They may take a little longer to trip, and usually stay tripped for a few seconds. Check the specs. If you can tolerate the little extra on resistance, this type is probably the most convenient if you regularly expect the power wires to be shorted.
There are electronic ways to do really fast overcurrent switches, but it doesn't sound like your application requires this level of complexity.
Your question is a little ambiguous, but I'll take it you're asking how to protect the transistor if you only have access to ground and the low side of the relay coil, not the power supply the other side of the coil is connected to.
Here are two possibilities:
- Use a zener diode. Connect the cathode to the collector and the anode to ground. This will conduct when the bottom side of the relay reaches the zener voltage. This voltage can be substantially higher than the relay supply voltage. It only needs to be below the maximum C-E voltage the transistor can handle.
This method has the advantage of turning off the relay faster compared to when using a typical flyback diode as shown in your question. This happens when the zener voltage is substantially higher than the relay coil supply voltage.
The zener max current must be rated for whatever the relay coil current it.
- Use a snubber. That's a resistor and capacitor in series between the collector and ground. The relay coil current time the resistor must not exceed the transistor C-E voltage capability. The capacitor has to be sized so that the total charge dumped on it from the relay coil flyback current doesn't cause more voltage than the transistor can handle.
Best Answer
Put the diode across the supply connections so that it is non-conducting, with a fuse ahead of it. The diode will conduct if the supply is reversed, and blow the fuse. There won't be any voltage drop.